The 29-year-old Facebook chief announced the stock and cash purchase on Wednesday, a deal that marries his social network of 1.2 billion active users with Whatsapp's 450 million users.

Asked about the price tag during an on-stage discussion at the Feb. 24-27 Mobile World Congress in Barcelona, Spain, Zuckerberg said WhatsApp was attractive as a company by itself, and as a strategic fit with Facebook.

“I just think that by itself it is worth more than US$19 billion (14-billion-euro),” said Zuckerberg, wearing a grey t-shirt, sneakers and black trousers.

“I mean it is hard to exactly make that speech today because they have so little revenue compared to that number,” he conceded.

“But the reality is that there are very few services that reach a billion people in the world. They are all incredibly valuable, much more valuable than that,” he added.

“I could be wrong. This could be the one service that gets to a billion people and ends up not being that valuable. I don't think I am.”

Other Android messaging applications such as KakaoTalk, Vine and WeChat were already bringing in two to three dollars a person “with pretty early efforts,” he said.

'Going to be a huge business'

“That shows that if we can do a pretty good job of helping WhatsApp to grow then this is just going to be a huge business,” Zuckerberg said.

“So even just independently I think it is quite a good bet.”

In partnership with Facebook, WhatsApp can focus on connecting “one, two, three billion people over the next however long that is going to take,” Zuckerberg said.